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What's the Difference Between Optical and Capacitive Encoders?

Nov

14

by Eric Rice

We offer different incremental encoder options for our stepper motors. Encoders are used to provide feedback to the motor drive or controller and improve overall performance of the step motor system. The following provides a summary of those encoder options.

Optical Encoders

Optical encoders have been around for a long time, are accurate and reliable, offer a wide range of resolutions, and are easy to work with. Optical encoders operate by passing light generated by an array of LEDs through a disc with slots in it to an array of optical sensors. The disc is mounted to the motor shaft, and as the shaft turns the slots in the disc alternatively pass or block the light emitted by the LEDs. This pulsing of the light as the shaft turns creates a series of digital pulses that provide position and speed information back to the motor controller. Figure 1 illustrates the basic operating principle.

Capacitive encoders utilize newer technology, offer similar benefits, and ultimately provide the same position and speed information as optical encoders, but the method of generating the digital pulses is based on a different principle. Figure 2 illustrates the basic operating principle of a capacitive encoder. An AC field transmitter emits a signal that is modulated by the metal pattern on a disc that is attached to the motor shaft. As the shaft turns, the sinusoidal metal pattern on the disc creates a signal modulation, due to varying capacitive reactance, that is repetitive and predictable. A field receiver on the other side of the disc receives this modulated signal and converts it into digital pulses for use by the motor controller.

Both the optical and capacitive encoders offered by Applied Motion Products are excellent choices for the most common motion control applications, however capacitive encoders do offer benefits in harsh environments. Optical encoders are susceptible to signal degradation/loss if exposed to dust, oil, or similar contaminants. To mitigate this susceptibility, we offer encoder covers to protect optical encoders. Capacitive encoders are generally immune to these environmental contaminants.

When using an encoder with an Applied Motion Products drive to perform Stall Detection or Stall Prevention functions, for example with an ST5, ST10, STAC5, or STAC6 series drive, we recommend any of the encoder options listed in the table below except for the BAA encoder option which is a single-ended variant of the CAA encoder. Single-ended encoders should only be used in installations where ambient electrical noise is well controlled and single-ended signals are acceptable. Whenever possible we recommend encoders with differential signals like the CAA, WAA, YAA/C, and ZAA/C encoder options.